I just got a 2007FP Dell monitor with height adjustable stand. When you adjust the height all the way down, the stand locks with the monitor so that you can transport the monitor safely. But suppose you want to unlock the monitor: you’re supposed to press the lockdown/release button on the bottom rear of the stand, but it gets stuck: you can’t press it!

Lockdown/release button on Dell 2007FP height adjustable stand
What you have to do is push down the monitor just a bit further (it’ll still move a little even though it’s locked down), and then the lockdown/release button will be released.